1. Field of the Invention
The invention relates in general to a fabrication method of an ion sensitive field effect transistor (ISFET), and more particularly to a fabrication method of an ion sensitive field effect transistor (ISFET) having a non-single-crystal-silicon-base substrate.
2. Description of the Related Art
Ion sensitive field effect transistor (ISFET) is a chemical sensor that combines principles of electrochemistry and microelectronics. It is provided for contact with a to-be-measured solution and measuring a concentration of a particular ion thereof. The ISFET is developed on the basis of metal oxide semiconductor field effect transistor (MOSFET) and by the enhancement of the MOSFET. The difference between the ISFET and MOSFET is that a gate of the MOSFET is of metal gate, while the ISFET includes an ion sensitive gate for contact with a to-be-measured solution. An operation principle of the ISFET will be described in the later part. In addition, since the ISFET includes advantages of high input impedance, low output impedance, high response speed, and the like, and also the ISFET features that fabrication technique is compatible with the MOSFET, the ISFET is rich in incomparable development potential.
FIG. 1 is a cross-sectional view showing a conventional ion sensitive field effect transistor. The ion sensitive field effect transistor (ISFET) 100 includes a monocrystalline silicon substrate 102, a source 104, a drain 106, a silicon oxide layer 108, a first metal electrode 110a, a second metal electrode 110b, a passivation layer 112, and an ion sensitive gate 113. The monocrystalline silicon substrate 102 is of lightly doped P-type (P−), and also the monocrystalline silicon substrate includes a front side of the substrate 102a. A method of the ISFET 100 fabrication is as follows. After a predetermined doped-region of the front side of the substrate 102a is defined, a step of doping the monocrystalline silicon substrate 102 with N-type impurities from the front side of the substrate 102a forms the two separated source 104 and drain 106 of heavily doped N-type (N+) in the monocrystalline silicon substrate 102. The source 104 and the drain 106 are not as thick as the monocrystalline silicon substrate 102. A predetermined channel region 107 is formed in a region between the source 104 and the drain 106 in the monocrystalline silicon substrate 102, and also the predetermined channel region 107 is near the front side of the substrate 102a. In addition, a silicon oxide (SiO2) layer 108 is formed on the front side of the substrate 102a, including a first contact hole 109a and a second contact hole 109b. Meanwhile, the first contact hole 109a and the second contact hole 109b partially expose the source 104 and the drain 106, respectively.
The first metal electrode 110a and the second metal electrode 110b are electrically couple to the source 104 and the drain 106 by the first contact hole 109a and the second contact hole 109b, respectively. And also the silicon oxide layer 108 is partially covered by the first metal electrode 110a and the second metal electrode 110b. The passivation layer 112 includes an opening 114 for exposing the silicon oxide layer 108 above the predetermined channel region 107. The ion sensitive gate 113 is formed above the silicon oxide layer 108 in the opening 113, which is provided for sensing ion concentration of a to-be-measured solution contained in the opening 114.
For example, when the opening 114 of the ISFET 100 is filled with a to-be-measured solution 202 containing positively charged ions, as shown in FIG. 2, the ion sensitive gate 113 will sense and measure the concentration of the positively charged ion in the solution 202, so that the ion sensitive gate 113 generates an interface variation of electric potential. At this time, under a circumstance of supplying a voltage difference Vds to the source 104 and the drain 106, there is also an electric current Ids flowing between the first metal electrode 110a and the second metal electrode 110b. Therefore, when there is the higher concentration of the positively charged ions in the solution 202, the ion sensitive gate 113 generates the more interface variation of electric potential. Comparatively, the electric current Ids flowing between the source 104 and the drain 106 would be larger. Consequently, the concentration of the positive ions in the to-be-measured solution 202 can be obtained.
One thing to note is that due to a pn-junction between the source 104 (N+), the drain 106 (N+), and the monocrystalline silicon substrate 102 (P−), an electric leakage phenomenon will occur. Thus, the measured electric current flowing between the first metal electrode 110a and the second metal electrode 110b is bias and not a substantial electric current. Therefore, there is a measurement error and the concentration of the positive ions in the to-be-measured solution 202 cannot be truly obtained. In addition, the monocrystalline silicon substrate 102 is very expensive, so as to increase a lot material cost.